A charge couple device (CCD) functions as an active matrix to absorb light and convert it to electrons. Such are used for example as the electronic film in digital still cameras(DSC).
Desirably, a charge couple device should absorb as much light as possible while, at the same time, minimizing leakage between pixels as much as possible.
A particularly perplexing problem of charge couple devices is referred to as dark current. Dark current is the undesired (noise/leakage) current which flows when no light is presented to the CCD array. Excessive dark current destroys the dynamic range of the CCD array when there is insufficient ability to distinguish between light and dark conditions.
The problem concerning dark current defects is especially critical in consumer digital still picture applications where cameras must function in both bright (short shutter speed) and low (long shutter speed) light conditions. This is typically not a factor in commercial (i.e. professional) in a studio setting.
It is known that hydrogen present in the CCD device will reduce dark current by passivating dangling bonds. However, while hydrogen could be used to solve the problem, keeping the hydrogen in the device is problematic since hydrogen diffuses very readily.
Attempts to prevent hydrogen diffusion have involved certain nitride films. For example, work on hot-electron effect mitigation with deuterium has shown that a barrier nitride layer limits loss of deuterium (D-2) from CMOS devices. Further, for solving this problem in CMOS devices, deposited nitride films containing some amount of hydrogen, trapped during the deposition process have been suggested. This hydrogen, in addition to any hydrogen annealed in before a barrier nitride deposition, acts as a hydrogen reservoir.
However, barrier nitride films suggested in the prior art are from plasma enhanced chemical vapor deposition (PECVD) or furnace low pressure chemical vapor deposition (LPCVD). In the case of a CCD device, neither of these process solutions is workable. PECVD films are too high in stress with the addition of sufficient hydrogen. While they prevent hydrogen diffusion which would limit dark current, their higher stress levels create dark current via a stress mechanism.
In addition, an attempt with a relatively thick (e.g. 1750 angstroms) hydrogen enriched PECVD reduced dark current, but the thickness of the film actually destroyed the necessary light transmissivity of the array. On the other hand, use of a relatively thin (e.g. 300 angstroms) hydrogen enriched PECVD nitride film had undesirable stress characteristics but adequate light transmissivity.
Furthermore, LPCVD films by comparison are very low in hydrogen content and also create integration problems due to the long heat cycle. Therefore, they are unable to provide a sufficient hydrogen reservoir. Additionally, the use of LPCVD barrier nitride adds a substantial thermal cycle which can change dopant profiles and allow for dopant deactivation.
Accordingly, a need exists to address the dark current problem of charge couple devices.